Genetics
- Monohybrid cross (Aa × Aa)
- genotypes 1 : 2 : 1, phenotypes 3 : 1
- Dihybrid cross
- phenotypes 9 : 3 : 3 : 1
- Hardy–Weinberg
- p + q = 1, p² + 2pq + q² = 1
- Independent genes
- multiply the probabilities
Biology: Genetics and Cell Biology: cram pack
Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.
1Formulas and methods
2Practice problems
1.GeneticsMedium
red-green colour blindness is X-linked recessive (allele b). A carrier mother, XBXb, has children with an affected father, XbY. The mother's gametes run down the side. Click the cell for an affected son.
♀ XBXb♂ XbY
- 1XB × Xb
- 2XB × Y
- 3Xb × Xb
- 4Xb × Y
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives black fur (dominant) or white fur; gene B gives tall plants (dominant) or short plants. Click the one cell for an offspring that is shows white fur and is homozygous for tall plants.
AaBb × AaBb
- 1ab × AB
- 2ab × Ab
- 3ab × ab
- 4ab × aB
- 5Ab × AB
- 6Ab × Ab
- 7Ab × ab
- 8Ab × aB
- 9AB × AB
- 10AB × Ab
- 11AB × ab
- 12AB × aB
- 13aB × AB
- 14aB × Ab
- 15aB × ab
- 16aB × aB
3.Cell biologyMedium
Which cell junction seals neighbouring epithelial cells together and stops fluid leaking between them?
- AHemidesmosome
- BDesmosome
- CGap junction
- DTight junction
4.Cell biologyHard
Knudson's "two-hit" hypothesis, based on retinoblastoma, explains that tumour suppressor genes:
- AAre always inherited in mutated form from both the mother and the father
- BUsually need both alleles to be inactivated before a tumour develops
- CCause cancer when one allele is overactive
- DAre only found in children
3Solutions
1.Genetics
Answer: Xb × Y
Punnett square: one parent's gametes run along the top, the other's down the side. Each cell joins the gamete above it with the gamete beside it, and every cell is equally likely. A recessive trait only shows when both alleles are recessive; for an X-linked trait, sons get their only X from the mother.
Fill the square
XBXb, XBY, XbXb, XbY
Find the cell
Xb × Y = XbY
Answer
Cell Xb × Y → XbY
2.Genetics
Answer: aB × aB
Punnett square: one parent's gametes run along the top, the other's down the side. Each cell joins the gamete above it with the gamete beside it, and every cell is equally likely. A recessive trait only shows when both alleles are recessive; for an X-linked trait, sons get their only X from the mother.
Genotype asked for
aaBB
Gametes
A homozygous genotype needs the same gamete from both parents, so only one cell in the 16 fits.
aB × aB = aaBB
Answer
Cell aB × aB → aaBB
3.Cell biology
Answer: D · Tight junction
Why this is the answer
Tight junctions join membranes of adjacent cells closely, blocking the paracellular route, for example in the gut lining and the blood–brain barrier.
Answer
D · Tight junction
4.Cell biology
Answer: B · Usually need both alleles to be inactivated before a tumour develops
Why this is the answer
Loss of one copy of RB1 is not enough; the second allele must also be lost. Children who inherit one mutant copy need only one more hit, so they often get bilateral tumours early.
Answer
B · Usually need both alleles to be inactivated before a tumour develops
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